This invention relates to a self-contained breathing apparatus, and more particularly to a closed-circuit positive pressure self-contained breathing apparatus for temporary use by a wearer in a noxious environment, such as is worn by a firefighter. (2) Description of the Prior Art
Self-contained breathing apparatus (SCBA's) are worn by industrial workers, and in particular firefighters, to provide a safe, respirable breathing condition while the user works in a hostile environment. Currently, breathing performance and service life rating of such apparatus are based upon user consumption at the rate of 40 liters per minute, wherein inhalation and exhalation reach peaks instantaneous flow rates of about 115 liters per minute. For firefighting duty, the National Fire Prevention Administration (NFPA) has defined new performance standards (for open circuit SCBA's) wherein peak instantaneous breathing rates exceed 200 liters per minute.
SCBA's are classified as open-circuit (where the user's exhalation is dumped from the system) or closed-circuit (where exhalation is returned to the system for subsequent reuse after carbon dioxide is removed and oxygen is added).
Although closed-circuit, self-contained breathing apparatus have existed longer than open-circuit types, there are some inherent disadvantages of closed-circuit systems which offset the substantial weight and size advantage they offer for extended duration. One disadvantage is the sluggish response of the system to the user's breathing requirements, particularly at high metabolic work rates. A second disadvantage is the design difficulty encountered in trying to create a constant positive pressure in the facepiece (positive pressure substantially increases the degree of respiratory protection to the wearer).